add comments to no-op downgrades

Author: connortsui20

library/std/src/sys/sync/rwlock/solid.rs

@@ -82,7 +82,8 @@ impl RwLock {
 
     #[inline]
     pub unsafe fn downgrade(&self) {
-        todo!()
+        // The SOLID platform does not support the `downgrade` operation for reader writer locks, so
+        // this function is simply a no-op as only 1 reader can read - the original writer.
     }
 }
 

library/std/src/sys/sync/rwlock/teeos.rs

@@ -43,5 +43,8 @@ impl RwLock {
     }
 
     #[inline]
-    pub unsafe fn downgrade(&self) {}
+    pub unsafe fn downgrade(&self) {
+        // Since there is no difference between read-locked and write-locked on this platform, this
+        // function is a no-op.
+    }
 }